This invention relates to a process and apparatus for regulating the concentration and distribution of oxygen in single crystal silicon rods, optionally doped with antimony or arsenic, prepared according to the Czochralski method and, in particular, to such a process and apparatus in which the gas pressure of the atmosphere over the melt is adjusted to a value in excess of 100 torr during the crystal growth process.
Oxygen in the silicon crystal may have both favorable and unfavorable effects. In various heat treatment processes conducted during the manufacture of electrical devices, the oxygen in the crystal may cause defects such as precipitates, dislocation loops and stacking faults. Alternatively, it may cause electrically active defects resulting in devices with inferior performance characteristics. The presence of oxygen in the crystal, however, increases the mechanical strength of silicon wafers and the crystal defects may improve the yield of conforming products by entrapping contaminants of heavy metals. Accordingly, the oxygen content of the silicon crystal is an important factor for product quality which must be carefully controlled in accordance with the requirements of the ultimate application for the silicon wafers.
In the conventional Czochralski method, the silicon melt is contained in a quartz crucible. During the process, some of the quartz dissolves into the melt as oxygen and silicon or some oxygen-silicon complex. A portion of the dissolved oxygen or oxygen-silicon complex migrates to the free melt surface and evaporates as silicon monoxide. Another portion of the dissolved oxygen or oxygen-silicon complex gets incorporated into the growing crystal. The remainder of the oxygen or oxygen-silicon complex is retained in the molten silicon.
As the crystal growth process continues, the free melt surface area remains constant while the melt level in the crucible decreases. As the melt level decreases, less of the surface area of the crucible is exposed to the melt and, therefore, less oxygen is incorporated into the melt. The net effect is that the bulk oxygen content of the melt decreases, which results in the production of silicon rods having axially decreasing oxygen contents.
The addition of antimony or arsenic as dopants causes the decrease in oxygen content to become more severe, an occurrence known as oxygen reduction. Oxygen reduction is the result of an increase in the vapor pressure of the silicon monoxide gas at the free melt surface which is caused by the presence of antimony or arsenic in the melt. This increase in vapor pressure causes the rate of evaporation of silicon monoxide to increase, resulting in an even lower bulk oxygen content in the melt.
Processes have been proposed for controlling the oxygen content in single crystal silicon and antimony doped silicon rods. For example, Seki disclosed in U.S. Pat. No. 5,423,283 a process for controlling the oxygen content in antimony doped single crystal silicon rods in which (i) the rotation rate of the crucible is gradually increased as crystal growth proceeds or (ii) a pulse-like change in the rotation rate of the quartz crucible is superimposed over a continuous increase in the crucible rotation rate. In either case, however, the atmosphere in the chamber is kept within the range of 7 to 38 torr.
Oda et al. disclosed in U.S. Pat. No. 5,131,974 a process for controlling the oxygen content of a single crystal silicon rod, prepared by the Czochralski method, whereby the pressure in the chamber and the supply rate of inert gas to the chamber are controlled with respect to the length of the crystal or the passage of time. This process provides for slowing the rate of evaporation of silicon monoxide by either decreasing the flow of inert gas into the chamber or increasing the pressure in the chamber.
Izunome et al., in the article "Control of Oxygen in Heavily Sb-Doped CZ Crystal by Adjusting Ambient Pressure," Mat. Res. Soc. Symp. Proc., Vol. 378 (1995), pp. 53-58, also disclosed that the evaporation of silicon monoxide from antimony doped silicon melts could be suppressed by increasing the pressure of the atmosphere over the melt. Antimony doped silicon rods pulled under a constant pressure of 100 torr were found to have a higher oxygen content than those pulled under a constant pressure of 30 torr.
The processes disclosed by Oda and Izunome, however, fail to address the problems caused by vapors which are trapped in the chamber as the pressure exceeds about 50 torr and which increase in severity as pressure increases. As the pressure over the melt increases, the unstable silicon monoxide vapors readily react to form silicon dioxide and silicon particulate. If this particulate comes into contact with the surface of the silicon rod or melt, a dislocation in the crystal, or a crystal defect, is formed. In addition, if not removed from the chamber, the trapped vapors and particulate will deposit on the surface of the view port window of the crystal puller. This deposit interferes with the crystal pulling process by obstructing the operator's view of the chamber and the silicon rod, as the process proceeds.